Sarcosomes and magnesium deficiency in ducks

Magnesium, electrolyte transport and coronary vascular tone

Coronary heart diseases (CHD) have high indices of mortality and morbidity. A number of CHD and myocardial ischaemic syndromes such as unstable angina pectoris, sudden death ischaemic heart disease, acute myocardial infarction and ventricular arrhythmias have been associated with losses of myocardial magnesium and potassium. Mg++ ions are essential for regulation of Na+ and K+ transport across cell membranes, including those found in cardiac and vascular smooth muscle cells. Mg++ activates an Na+-K+-ATPase pump which in turn plays a major role in regulating Na+-K+ transport. Loss of cellular Mg++ results in loss of critically important phosphagens: MgATP and creatine phosphate. Thus, under conditions where cellular Mg++ is depleted (e.g. hypoxia, ischaemia, anoxia), the Na+-K+ pump and phosphagen stores will be compromised, leading to alterations in resting membrane potentials. Cellular Mg++ depletion has been found to result in concomitant depletion of K+ in a number of cells, including cardiac and vascular muscles. The consequences of these events are often production of cardiac arrhythmias. Myocardial and vascular injury lead to disturbances in electrolyte transport across cell membranes, whereby Na+ and Ca++ uptakes are enhanced and, just prior or concomitantly, Mg++ and K+ are lost. Such electrolyte disturbances often lead to necrotic foci. Considerable evidence has accumulated to indicate that the extracellular concentration of Mg++ is important in control of arterial tone and blood pressure via pressure via regulation of vascular membrane Mg++-Ca++ exchange sites. A reduction in the extracellular Mg++ concentration can produce hypertension, coronary vasospasm and potentiation of vasoconstrictor agents by allowing excess entry of Ca++; concomitantly, the potency of vasodilator agents is reduced. Alterations in vascular membrane Mg++ results in arterial and arteriolar membranes which are 'leaky', thus contributing to the cellular reduction in K+ and gain of Na+ and Ca+. Alterations in extracellular K+ or Na+ concentrations over physiological ranges, in the face of a Mg++ deficit, can exacerbate the coronary vasospasm noted with reduction in only extracellular Mg++. Since free Mg++ ions are necessary for maintaining Ca+ ions (both plasma membrane-bound and sarcoplasmic reticulum membrane-bound via Ca++ ATPases), intracellular free Mg++ would rise in conditions which result in cellular loss of Mg++, thereby exacerbating and contributing to elevation of blood pressure and coronary vasospasm.(ABSTRACT TRUNCATED AT 400 WORDS)

Nucleotide concentrations in cellular tissues during potassium and magnesium deficiency

Potassium deficiency reduced the concentrations of ATP and GTP in rat thigh muscle but not in liver. The concentrations of UTP, CTP and the UDP-sugar derivatives with glucose, galactose and N-acetylglucosamine in liver were all increased. Magnesium deficiency, however, had little effect on the nucleotide concentrations in liver.

Changes in cellular composition during magnesium deficiency

Mg deficiency increased the water content of the liver, kidney, heart and thigh muscle in the rat and decreased the proportion of nitrogen in the dry matter of the same tissues. Changes in the concentration of metals also occurred. 2. Cellular fractional indicated that the Mg and K depleted in liver occurred primarily in the heavy-mitochondrial and microsomal fractions respectively. The calcification of liver and kidney was due to preferential deposition of Ca in the heavy-mitochondrial fraction. 3. The proportion of cellular nitrogen present in the heavy-mitochondrial and microsomal fractions was markedly decreased in the liver and kidney of the Mg-deficient rats, and the proportion in the supernatant fraction increased. 4. Electron microscopy revealed that the cross-sectional areas of liver cells and all their mitochondria were decreased in Mg deficiency. The number of mitochondrial per cell was decreased even more severely and the average area of a mitochondrion was greater in deficient rats than in control animals. 5. The significance of these observations is discussed in relation to the location of the primary metabolic disturbance during Mg deficiency.

Magnesium deficiency and cardiac disorders

Magnesium deficiency can occur in congestive heart failure, after diuresis with furoxemide, ethacrynic acid and mercurials, and with digitalis intoxication, diabetic acidosis, acute and chronic alcoholism, delerium tremens, cirrhosis, malabsorption syndromes, protracted postoperative cases, open heart surgery, the diuretic phase of acute tubular necrosis, and with hypoparathyroidism, primary aldosteronism, juxta-glomerular hyperplasia and pancreatitis. Two cases of serious ventricular arrhythmias associated with magnesium depletion are described. Clinical manifestations are vague but center around neurologic symptoms such as weakness, tremors, stupor, coma, nausea, vomiting and anorexia. Serious cardiac arrhythmias also occur with magnesium depletion. Magnesium appears to be very useful in hypomagnesemic or digitalis-toxic tachyarrhythmias. Magnesium may also be valuable in normomagnesemic tachyarrhythmias. Ten to fifteen milliliters of a 20 percent magnesium sulfate solution, given intravenously over 1 minute, followed by a slow 4 to 6 hour infusion of 500 ml of 2 per cent magnesium sulfate in 5 per cent dextrose in water is recommended. Recurrence of arrhythmias is common and a second infusion of magnesium sulfate may be necessary. Hypermagnesemia occurs frequently in renal insufficiency, and magnesium therapy may then be contraindicated. Serum levels above 5.5 meq/liter should be avoided. Loss of deep tendon reflexes and a decrease in respiratory rate can be used as guides to magnesium therapy. A plea is made for frequent analysis of serum magnesium so that more knowledge can be gained regarding this important biologic element in cardiovascular disorders.

CARDIAC NECROSIS AND CALCIFICATION IN EXPERIMENTAL MAGNESIUM DEFICIENCY. A LIGHT AND ELECTRON MICROSCOPIC STUDY

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